How to Specify
Pumps for Transferring
Acidic, Caustic, Abrasive
and Toxic Solutions
Acids, Caustic solutions
CHEM-GARD Horizontal Centrifugal Pump, FLEX-I-LINER
Sealless Self-Priming Peristaltic Pumps, SUMP-GARD
Thermoplastic Vertical Pump
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Vanton Pump and Equipment Corp.
Ever tightening regulations and heightened
environmental responsibility have driven industrial
and municipal facilities to rethink leak prevention,
particularly when handling acidic, caustic and
abrasive solutions that cause packing and
mechanical seals to fail, and toxic solutions that
demand failsafe containment. To transfer these
solutions, operations and engineering management
is relying increasingly on sealless, non-metallic
pumps. Given the variety of types offered, recent
advances in pump design and the myriad of
fluid-contact materials available, this article
attempts to update specifiers with guidelines
needed to select the optimum sealless pump
according to individual application parameters.
Heightened awareness of environmental, safety and maintenance issues
in recent years has made process-fluid leakage a paramount concern in
chemical processing plants and municipal wastewater facilities. The
ability of sealless pumps to prevent or minimize leakage has made them
the often-preferred choice for pumping of acidic, caustic and abrasive
solutions that cause seals fail, as well as toxic solutions that require
absolute containment. After years of being considered niche products
for specialty applications, sealless pumps have become mainstream.
Pump design trends with respect to the handling of corrosive and
abrasive fluids, point to the use of sealless configurations to minimize
leakage, and the specification of nonmetallic, chemically inert, and
abrasion-resistant materials of construction to ensure resistance to
acids, caustics, salts and other aggressive fluids. To select the optimum
sealless pump for an application, one must become familiar with the
technical and economic issues involved in the pump-selection process,
the available pump types, and the various nonmetallic materials that are
suitable for the service.
Defining a sealless pump
The Hydraulic Institute (Parsippany, NJ) defines a sealless pump as one
in which the impeller shaft is completely contained in a sealed,
pressurized vessel (called the containment shell) that contains the
process fluid. Leakage of the pump fluid into the surrounding
environment is prevented by the exclusive use of static, rather than
dynamic, sealing technology. Only two pump designs meet this
definition: the magnetic-drive pump (MDP), in which the impeller shaft
is driven by a magnetic-coupling arrangement, and the canned-motor
pump (CMP), which features a rotating magnetic field within the motor
stator. The Hydraulic Institute definition is therefore valid, but
unnecessarily limiting. For the purpose of this article, a sealless pump
is defined as one that does not use packing or mechanical seals to
isolate the process fluid.
This broader definition permits consideration of a larger group of pump
designs. There is, however, no intent to imply that these other
sealless-pump designs eliminate the risk of hazardous or toxic
emissions. This discussion is based on a search of commercially
available designs with a focus on ones whose fluid-contacting parts are
made of nonmetallic materials. No implication is made that this search
has been all-inclusive and that all commercial products have been
A wide variety of nonmetallic materials is available for the construction
of wetted pump components used in corrosive-fluid applications.
These include thermoplastics, thermosets and elastomers. A general
knowledge of the characteristics of each class of material is helpful in
selecting the proper one for a particular application. Table 1
summarizes the significant physical characteristics of the rigid plastics
and Table 2 compares the elastomers used most often in aggressive
Sealless pump types
If the elimination of the mechanical seal is an important factor in pump
selection, then there are other pump types, in addition to the MDP and
CMP, that deserve consideration. But the choices narrow as application
and service requirements become more stringent.
For instance, if the pump must be chemically resistant to the process
fluid, the scope becomes limited to units with wetted components made
of the stainless steels, high and exotic alloys and nonmetallics. If the
analysis is restricted to applications where non-metallic materials are
considered ideal for providing the required chemical resistance, then
the options are limited to five commonly used sealless pumps, whose
wetted parts are made of thermoplastic, thermoset and elastomeric
materials. These configurations are the magnetic-drive pump, the
coupled wet-pit vertical sump pump, the flexible-tube pump, the
flexible-liner pump and the controlled-volume diaphragm pump.
The next segment of this article briefly describes the design principles of
each of these nonmetallic pump configurations. Subsequently, the
critical design factors are detailed.
Magnetic drive pump
The magnetic-drive centrifugal pump (Figure 1) offers flows to
approximately 1,000 gal/min (227 m³/hr), and heads up to 350 ft (107
m). A nonmetallic containment shell or can, encloses and statically
seals the entire impeller-rotor assembly, as well as the pumped fluid.
This pump has two shafts. The driven shaft is located in the liquid end
of the pump and is supported by sleeve bearings. The impeller and the
inner magnet are mounted on this shaft. The other shaft, called the
driving shaft, is either a close-coupled motor shaft or one that is
supported by antifriction bearings in the pump-bearing housing. The
driving magnet surrounds the containment shell and is mounted on this
Coupled, vertical sump pump
The coupled, wet-pit vertical sump pump (Figure 2) discussed here is a
non-metallic sump pump that does not employ shaft-sealing
arrangements. It offers flow rates up to 4,000 gal/min (908 m³/hr) and
heads up to 350 ft (107 m), and can be used in sumps as deep as 50 ft
In this class of pumps, the impeller's hydraulic design is of a radial type,
and the pumped fluid exits through a separate discharge pipe rather
then coming up through the column. The pumped fluid that fills the
column is returned to the sump through radial leakage holes in the
column. The pressure in the column is atmospheric at the uppermost
leakage hole situated below the manhole cover, so that the liquid level
in the column remains below the point at which the shaft penetrates the
pump support plate — hence a shaft liquid seal is not required. A
dynamic vapor seal is employed in many cases, at the juncture between
the shaft and cover plate to prevent escape of corrosive fumes that
might attack the motor and its support bracket. The open-line shaft
bearings are typically product-lubricated.
Vertical pumps are furnished in a variety of configurations, including:
cantilevered-shaft designs, which are suitable for dry-running
conditions; vortex-recessed impeller designs, for handling fluids with
solids or stringy debris; and segmented-shaft designs, for installation in
extremely deep sumps, or for installations of tall pumps in
The flexible-tube pump (Figure 3) does not use mechanical seals.
Instead, the fluid is contained within the smooth walls of an elastomeric,
tubular structure and is moved forward as the tube is squeezed by a
rotating element. As the squeezed tube returns to it natural shape, the
vacuum produced by the displaced fluid draws more fluid into the tube.
Pumping is achieved by a gentle, peristaltic action that allows for a
controllable flow of the fluid trapped between the two contact points on
the inside of the tube. This configuration requires no seals, glands or
valves. Models are available with flow rates up to 200 gal/min (45
m³/hr) and differential pressures to 200 psi (1379 kPa).
The flexible-liner pump (Figure 4) moves the fluid forward peristaltically
via an eccentrically mounted rotor that applies pressure from the inside
of a flexible liner within the pump body. Fluid is contained within a
channel-like cavity formed by the outer surface of this elastomeric liner
and the inner surface of the thermoplastic pump body. The rotor is
mounted on an eccentric shaft that oscillates within the liner and
progressively moves the sealing contact point between the liner and
pump body, effecting a squeegee action on the trapped fluid.
The flexible-liner pump is available in either close-coupled or
pedestal-mounted configurations. It is self-priming and has no stuffing
boxes, glands, valves or gaskets. Flow rates range from 0.30 to 40
gal/min (.075 to 9 m³/hr), with a maximum differential pressure of 30 psi
(207 kPa). The pump can be driven by an electric, gasoline or air motor.
The controlled-volume diaphragm pump (Figure 5) has a flexible
diaphragm that directly contacts the process fluid. This diaphragm also
acts as a seal between the drive mechanism and the pumped liquid.
Many design configurations are available. The diaphragm can be driven
mechanically, hydraulically, pneumatically or electromagnetically. The
pumps are available in single-, double- and multiple-diaphragm
configurations. All diaphragm pumps are sealless and self-priming, and
can be run dry without causing damage. Flow rates of 200 gal/min (45
m³/hr) and outlet pressures of 100 psig (6.89 bar) are common.
Figure 6 - This magnetic-drive,
close-coupled pump is shown with wet end
opened and the solid,
molded-thermoplastic casing, impeller
and bearing housing exposed. All
components are produced from
polypropylene or polyvinylidene
fluoride. In this design, no metal
comes in contact with the pumped fluid.
Figure 7 - Cut-away view of a vertical
centrifugal sump pump made of
polyethylene showing rugged ribbed
column construction, molded
thermoplastic casing and impeller, and a
thick-sectioned nonmetallic sleeve that
isolates the stainless steel shaft from
the fluid. There are no bearings inside
the fluid cavity and no seals, except
for the nonmetallic vapor seal in the
cover plate, which protects the motor
bracket assembly from corrosive fumes.
Figure 8 - In a flexible-tube pump, the
tube passes through the pump body, where
a set of rotating rollers compresses the
tube and enables flow.
Figure 9 - Pedestal-mounted
flexible-liner pump with all
fluid-contacting parts made of
Figure 10 - The flexible-liner pump has a
wide choice of body-block and liner
materials. Liner materials include
natural or butyl rubber, neoprene,
monomer (EPDM), Buna-N and
chlorosulfonated PE. Liners are shown
with various pump casing materials,
which are: Left to right (top row): PP,
Teflon, Rulon; (bottom row): PE,
stainless steel, PP.
CRITICAL DESIGN CHARACTERISTICS
Magnetically driven pumps
In the magnetic-drive pump, the pump casing is a pressure-containment
component whose strength is a significant characteristic. Mechanical
strength is also an important factor to consider when determining the
flange loading that can be carried by the pump. MDPs are available
with metal casings that are lined with thermoplastics or constructed of
fiberglass-reinforced thermoset resins and solid, molded
thermoplastics. Some solid thermoplastic designs also incorporate
cast-iron structural supports to provide enhanced pressure-containing
and nozzle-load capabilities that match those of metal pumps meeting
ANSI process-pump standards.
When considering the use of metal casings lined with corrosion-resistant
thermoplastics, special consideration of the service conditions is critical.
The following questions should be addressed when selecting the
special material: How will the lining hold up under the flow conditions?
How abrasion-resistant is the lining material? How significant is the
danger of wear or pinholing, which might lead to corrosion of the metal
or contamination of the product? These concerns become less
significant as lining thickness increases, or with designs that utilize
thick-sectioned, replaceable wet-end components.
Driven shaft: This component features a stainless-steel or high-alloy
shaft that is completely encapsulated, or sleeved, in a chemically inert,
nonmetallic material, such as polypropylene (PP) or polyvinylidene
fluoride (PVDF). This design provides strength, and allows one to select
the thermoplastic material based on the required corrosion resistance to
the chemicals being pumped. Some pumps utilize a ceramic shaft,
which eliminates concern about chemical resistance or product
Bearing carrier or housing: This structural component houses the
wet-end bearings. The bearing housing features either solid
thermoplastic construction (Figure 6) or a plastic-lined metal
component. The same precautions noted for selecting the
thermoplastic with suitable corrosion resistance for pump casings apply
here. Bearings that are immersed in the process fluid are generally
furnished in non-metallic materials, such as ceramics, carbon-filled
polytetrafluoroethylene (PTFE), or silicon carbide
Containment shell (the can): The containment shell, like the casing,
must withstand high pressure. Its material of construction is selected
according to the required corrosion resistance and mechanical strength.
Most nonmetallic MDPs utilize a two-layer can, whereby the inside can
— the one in contact with the corrosive fluid — is made from chemically
inert fluoropolymers. The outer can, which is not in contact with the
aggressive fluid, and mainly provides mechanical strength, is generally
furnished in a fiber-reinforced-plastic resin composite. Sometimes, the
outer can is of metallic construction. In addition to providing corrosion
resistance, the nonmetallic cans generate less heat than metallic ones.
Heat is generated by wet-end-bearing friction, by hydraulic losses due
to the rotation of the inner magnet in the fluid and by eddy currents on
the surface of the can, caused by the rotating magnetic field in the
coupling. This heat is carried away by circulating some of the pumped
fluid between the outside of the inner magnets and inside of the can.
The circulated fluid leaves the can at a higher temperature than that at
which it entered, and the temperature of the liquid in the can is actually
higher than the process temperature. Non-magnetic cans effectively
avoid troublesome eddy currents and the associated heat generation
that reduce pump efficiency and reliability
Driven magnet: The inner magnet is constructed of rare-earth metals
such as samarium cobalt or neodymium. To provide the chemical
resistance required, these magnets are completely encapsulated with
PTFE, PVDF or PP.
Coupled, vertical sump pumps
These chemical pumps, intended for corrosive services, are available in
PP, polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC),
PVDF and fiberglass-reinforced plastic (FRP). Wetted parts of the pump
assembly consist of the portion of the pump shaft situated below the
pump mounting plate (also called a coverplate), the shaft sleeve
bearings, the casing, the impeller and the pump column, which
structurally supports the immersed casing and shaft (Figure 7).
This pump is available in various design configurations. Typically, the
motor mounts above the mounting plate. In cantilevered designs,
which are recommended for use where dry running may occur or
where acceptable wet-bearing lubrication is not attainable, the pump
shaft is supported by anti-friction bearings above the mounting plate.
There are no bearings immersed in the fluid. These pumps are limited
to sump depths of approximately 5 ft (1.5 m), but with tail pipes they
can effectively be used in slightly deeper sumps.
Motor-support configurations: For light-duty, low-cost and intermittent
service, the pump motor may be mounted directly on top of the
thermoplastic mounting plate. On the more rugged, heavy-duty
designs, the motor is mounted on a cast-iron, motor-support pedestal.
This pedestal elevates the motor above the mounting plate, as well as
above the vapor seal in the coverplate, thereby protecting the motor
from exposure to trace amounts of corrosive fumes that might escape
through the seal. The elevation also provides the vertical height
necessary to utilize separate anti-friction bearings for extra shaft support
Shaft configurations: When selecting a vertical pump, the user should
note that reliability is, in part, dependent on whether the shaft is
supported by means of a cantilever or by a wet bearing.
In wet-bearing designs, the shaft is supported from above the mounting
plate using antifriction, grease-lubricated bearings in the pedestal, and
is additionally supported from below the mounting plate with
product-lubricated sleeve bearings.
Material selection is the key to protecting these bearings from the fluid.
The choice for the outer bearings includes ceramic, silicon carbide,
siliconized carbide, carbon-filled PTFE and glass-filled PTFE. For
extreme conditions, the shaft journals (or inner bearings) are
constructed of ceramic materials.
It is important to note that wet bearings must remain wet in order to
operate properly. Serious damage can occur if wet bearings are
allowed to run dry. Sealless sump pumps provide bearing lubrication
by tapping fluid from the pump discharge and routing it to each bearing.
This product lubrication method is required because the upper wet
bearings are not flooded by the liquid in the sump when the liquid level
is low. Furthermore, when the pumped fluid contains solid particles
that can damage the wet bearings, an independent clean-water flush
must be incorporated in the pump design.
By definition, a cantilevered sump pump must provide all shaft support
from above the mounting plate. This design stipulation stems from the
fact that the impeller is overhung on the pump shaft and is not
supported from below the mounting plate. Additionally, in the
cantilever sump pump, the hydraulic radial and axial loads must be
sustained by the anti-friction bearings located in the motor pedestal.
The shaft must be rugged and of a large-enough diameter to minimize
shaft deflection and stresses caused by these hydraulic loads. It also
requires heavy-duty antifriction bearings that can carry the larger loads
imposed by a cantilever design.
Unlike horizontal, overhung designs, these vertical pump configurations
may require different bearings and shaft diameters as the pump length,
and hence the overhung length, changes. Corrosion and potential
metal contamination of the fluid can be prevented by constructing the
shaft of Type 300 Series stainless steel, and sleeving the shaft with a
thermoplastic material. If metal contamination is not a problem,
superior corrosion resistance can be achieved by upgrading from
stainless steel to more-costly metal alloys. In any case, it is important to
select the shaft and encapsulation material on the basis of cost,
anticipated service life and the ability to resist attack from the process
Flexible-tube or flexible-hose pumps were originally designed for
low-flow metering applications, and have gradually worked their way
from the laboratory into production-level applications. They handle
slurries, high-viscosity fluids and abrasives, can be run dry, and have
excellent self-priming capabilities. Pulsations are present in the
discharge line due to the nature of the pumping mechanism. The
following section discusses the three major assemblies of the flexible
tube pump — the head, tubing and drive.
Pump head: The pump head contains the tubing, the roller or
shoe-drive assembly that traps the liquid in the tube, and the casing that
houses these components. The rollers or shoes squeeze the outside of
the tube against the bore of the casing, trapping the liquid between the
squeeze points (Figure 8). Since the corrosive liquid does not contact
the head under normal operating conditions, the head on larger
industrial units is usually constructed from cast-metal components.
Smaller, lighter-duty units, which are often used for metering
applications, may have heads constructed of rigid plastic. Duplex
systems, in which two heads utilize one drive, provide higher flows and
reduced pressure pulsation.
Tubing: A variety of elastomeric materials, including polyurethane,
chlorosulphonated polyethylene, and nitrile, butyl and natural rubber,
are available for tubing construction. Some manufacturers offer
proprietary materials, such as cord-reinforced tubing or special natural
and synthetic rubber or plastic composites designed for specific service
requirements. Although numerous references offer information on the
chemical resistances and temperature limitations of these standard and
proprietary elastomeric materials, they often fail to account for variables
such as fatigue, repetitive flexing and similar factors
The service life of a flexible tube pump is highly dependent on the
specific material selected for the tubing. Since the pumped fluid is
totally contained inside the tubing, the fatigue life and the maximum
pressure of these pumps are dictated by the material characteristics and
the fluids being pumped. Flexible-tube-pump hoses undergo many
cycles of compressive and tensile stress. Even if the original tubing
were replaced by tubing with similar characteristics, the pump might
not perform as it did with the original material, unless that material were
provided by the original supplier.
Drive: Most flexible-hose pumps operate at a shaft speed below the
motor's synchronous speed. This is achieved by employing reducing
gears or a variable-speed drive. The use of a variable-speed drive
permits the pump flowrate to be varied so that specific metering
requirements are met. This also helps extend the fatigue life of the
Flexible-liner pumps are available in close-coupled configurations with a
C-face motor, and frame-mounted designs coupled to a foot-mounted,
horizontal motor (Figure 9). These units are self-priming, can be run
dry, and can dependably handle slurries and viscous liquids. Like
flexible-tube pumps, flexible-liner pumps also tend to produce pressure
pulsations. At selected speeds, however, the pumping action is gentle
enough to prevent the settling out of suspensions and provide for the
effective handling of latex emulsions and similar materials. Duplex
designs are available for higher flows and for reducing the pulsation
tendency. The use of two opposing eccentric shafts oriented 180 deg
out of phase cancels pumping pulsations generated within each fluid
cavity. The user should provide flexible-hose suction and discharge
connections to avoid transmitting piping loads to the pump nozzles and
Flexible-liner pumps operate at 1,800 rpm or less and are available with
variable-speed drives. Unlike some positive-displacement pumps,
flexible-liner pumps can be operated at zero flow for short periods of
time, but it is not recommended that the differential pressure exceed 30
psi (207 kPa) for continuous service on most sizes. The major
components consist of the liner, body block, rotating assembly, cover
plate and bearing frame.
The two components for which material selection is critical are the body
block and the liner. These are the only components in contact with the
Rotating assembly: The rotating assembly of the flexible-liner pump
consists of an eccentric rotor that is mounted on an overhung,
frame-mounted shaft, and is completely isolated from the pumped fluid.
The liner acts as a joint gasket between the pump body and cover
plate, and between the body and the bearing frame. Since the
aggressive fluid does not contact the rotor, the rotor does not require
special materials of construction. As the rotor oscillates within the liner,
it creates a sealed, rolling contact point between the inside surface of
the body block and outer surface of the liner. This imparts a progressive squeegee action on the trapped fluid
Body block: The body block contains the suction and discharge nozzles,
and is considered to be the pump casing. Constructed from rigid
thermoplastics, the body block is sandwiched between the two external
flanges of the liner, which act as gaskets. The interior surface of the
bore is in direct contact with the fluid, making material selection critical.
Standard units provide a choice of ultra high-molecular-weight
polyethylene (UHMWPE), PP and PTFE (Figure 10)
Flexible liner: The liner is a thick-walled, molded elastomeric
component that can readily be replaced in the field without the use of
special tools. Only the outside circumference of the rugged unit is in
contact with the pumped fluid. Its cross section forms an "H" pattern —
the vertical legs of the "H" act as static gaskets between the body block,
the cover plate and the bearing frame. It is this feature that makes the
flexible-liner pump a "sealless" pump. The flanges on this liner are
pressed to the sides of the body block by concentric grooves on the
pedestal assembly and cover plate, isolating the fluid within the formed
Although this type of pump is well suited for pumping clear or viscous
liquids and slurries with soft solids, it may experience difficulty with
fluids that contain hard or sharp solids. The wide choice of liner
materials available, and the ease with which liners can be changed,
makes it economically feasible to utilize a single flexible-liner pump for
Figure 11 - Shown is a cutaway of a
controlled-volume diaphragm pump with
double diaphragms and check valves
The controlled-volume diaphragm pumps are widely used with viscous
liquids, abrasive slurries, shear-sensitive liquids (such as paint) and
fluids containing small, suspended solids. The diaphragm's relatively
low oscillation frequency and low velocity are gentle on the fluid being
The air-operated, duplex nonmetallic pump is the type most widely used
in the CPI. This configuration features two diaphragms linked by a
common shaft (Figure 11). An air-valve mechanism controls the
oscillating stroke of the shaft. The suction stroke that draws pumped
liquid into the pumping chamber doubles as the discharge stroke on the
opposing diaphragm, which expels liquid out of that pumping changer.
Like many positive-displacement pumps, the diaphragm pump has a
pulsating discharge pressure. Pulsation modulation is achieved via
dampeners and flexible-hose connections.
In addition, diaphragm pumps are self-priming, and can be run dry.
Air-operated units may be submerged if all of the pump components
are corrosion resistant, and if the driving air can be vented through the
pumped liquid. Automatic, variable-speed capability is achieved by
controlling the inlet-air pressure and flowrate. The major components
of the wet-end assembly include the body, diaphragm, and suction and
discharge check valves and valve seats.
Body: The body of the controlled-volume diaphragm pump is the pump
casing. The joint between the body and the diaphragm separates the
wet-end pumping chamber from the mechanical power end. Materials
of construction for this component include PP, PVDF and PTFE
Diaphragm: The flexing of this elastomeric component is responsible
for the pumping action. The material of construction for this critical
component should be selected on the basis of its resistance to the
aggressive fluid that is being handled. Options include neoprene,
polyurethane, PTFE, Buna-N, EPDM, fluoropolymers, and
Check valves: The check valves control the flow of liquid into and out of
the pumping chamber, and are exposed to the corrosive pumped liquid.
Materials of construction are similar to those used for the diaphragm.
There is one check valve at the inlet and one at the exhaust of each
liquid pumping chamber
Check-valve seats: During the discharge stroke of the diaphragm, the
check valve at the inlet of the pumping chamber is pushed against the
seat at the pumping-chamber inlet. Likewise, during the suction stroke,
the check valve at the discharge end of the pumping chamber is pushed
against the seat at the respective location. Since the check-valve seats
are exposed to the pumped fluid, they must be chemically resistant.
They can be supplied in the same variety of elastomers as the
The selection process
The selection of the ideal sealless thermoplastic pump for a particular
application requires a thorough awareness of the application
requirements, including the ability of the pump to handle the required
flow and differential pressure, and what materials will perform reliably in
the service. When selecting a material of construction, three criteria are
used to simplify the choice of a specific thermoplastic: The maximum
fluid temperature, the desired abrasion resistance of the pump and the
chemical inertness of the pump material to the process fluid.
In the 1950, Vanton developed a revolutionary all-plastic pump for use in conjunction with the first heart-lung device. The design limited fluid contact to only two non-metallic parts: a plastic body block and a flexible liner. This was the birth of our Flex-I-Liner rotary pump. Its self-priming sealless design made it an industry standard for the handling of corrosive, abrasive and viscous fluids as well as those that must be transferred without contaminating the product. Vanton now offers the most comprehensive line of thermoplastic pumps in the industry.
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Vanton Pumps (Europe) Ltd.
Unit 4, Royle Park
Congleton CW12 1JJ